Since the advent of motorized vehicles it has been desirable to measure and display various characteristics of vehicle performance and operation. While there are many different types of engine-driven vehicles, including automobiles, trucks, aircraft, and boats, the nature of the parameters to be measured and displayed is fundamentally similar, and many are common to all of the aforementioned applications, such as, for example, vehicle speed, engine speed, engine oil pressure, coolant temperature, battery voltage, and remaining fuel.
The mechanism for measuring and displaying these and other parameters generally comprises a sender and a gauge attached thereto. The sender measures the specific parameter of interest and converts that quantity into an electrical signal. The gauge receives the electrical signal from the sender and converts it into a human-readable display, such as, for example, a movement of a pointer across a dial face. Typically, the gauge includes a housing and a display mechanism. The housing, which has a viewing window, protects the display mechanism from dirt and moisture, while the display mechanism receives the electrical signal generated by the sender, converts the signal into a human-readable format, and displays it, such as, for example, by moving a pointer across a printed dial face which can be viewed through the housing's window.
Historically, these systems have enjoyed very widespread use and acceptance. Most engine-driven vehicles have at least one such sender and gauge pair, and many, such as dual engine boats, have upwards of seven. However, despite their popularity, traditional sender and gauge systems present certain inherent drawbacks with respect to responsiveness and precision. Traditional gauges employ a display mechanism known as an air core movement in order to move the pointer across the dial face. In general, an air core movement operates by running current through one or more coils of wire, which sets up an electromagnetic field. Variations in sender resistance or voltage change the magnitude and/or direction of the coil current and thus the resulting magnetic field, such that a magnet mounted on a shaft within this field will turn the shaft as it orients itself within the field. A pointer mounted on the shaft is consequently made to turn with the magnet in a one-to-one correspondence, moving the pointer across a dial face.
Lack of precision in display mechanisms with air core movements arises due to lack of precision in the coil wire and magnet. Accuracy typically ranges from three to five percent error, with some air core movements being off by as much as ten percent. Also, air core movements typically have hysteresis, such that they respond differently to a specific sender value, depending on whether the movement was moving clockwise or counterclockwise prior to receiving the sender value. Air core movements are also typically non-linear with respect to linear sender input, such that dial faces must be drawn with increasing or shrinking ranges, making them hard to read. Non-linear senders, such as those used for sensing temperature, add to the inaccuracy, as the dial graphics must try to account for eccentricities of both the sender and the air core movement.
Furthermore, the limited torque generated by a typical air core movement restricts the variety of pointer masses which may be successfully accommodated by the display mechanism, making the display mechanism susceptible to vibration such that hard jolts to the gauges caused by, for example, a boat traveling through rough seas, can make the pointers vacillate considerably across the dial face. While pointer movement is typically damped by means of liquid silicone, to prevent excessive jitter, such damping makes the response sluggish, and the silicone may leak out during shipping.
An additional drawback of air core movements is that the movement within each gauge must be specifically configured to interpret the signal received from the sender, such that it is incapable of being used with a sender of a different scale. For example, traditional fuel senders include, among others, senders which run linearly from 0 to 90 ohms across an Empty to Full range, as well as senders which run linearly from 240 to 33 ohms across the same range. A traditional fuel gauge has a display mechanism that associates specific ohm readings from the sender, such as, for example, 45 ohms, with specific fuel quantities. Consequently, a display mechanism which is designed to operate with the 0 to 90 ohm sender will give inaccurate readings when used with a 240 to 33 ohm sender. This characteristic applies to all traditional sender and gauge pairs, resulting in inconvenience, expense, and even re-work when gauges and senders are not properly matched.
Those skilled in the art will recognize that some traditional gauges, such as speedometers and tachometers, have attempted to address the configuration issue by utilizing dip switches or potentiometers to permit some flexibility in how the display mechanism within the housing interprets the incoming sender signal. These dip switches and potentiometers permit accommodation for a variety of sender characteristics, such as pulses per engine revolution or tire sizes or axle ratios. However, the presence of these configuration options exacerbates one final problem with traditional gauge and sender pairs which pertains to the lack of sealing of the housing.
Vehicle applications include a variety of environments which may be hostile to gauges generally and to their display mechanisms in particular, such as, for example, marine or construction vehicles. As noted previously, the housing portion of traditional gauges serves as a primitive means of protecting the display mechanism from environmental effects. The presence of dip switches or potentiometers dictates that these housings entertain a breach to make the configuration accessible to the gauge installer. Since housings already entertain a breach for servicing the incandescent bulb which provides backlighting, adjustable display mechanisms increase the opportunity for introducing environmental debris such as moisture, salt, or dirt. Hence the vehicle manufacturer must take particular care to shield the gauges from any debris which could negatively affect the performance of their display mechanisms. Likewise, the vehicle owner/operator must provide substantial maintenance to the gauges to remove accumulated debris which may shorten gauge life.
There is thus a need to provide a display apparatus that eliminates or minimizes one or more of the problems as set forth above.